Mutations in the gene encoding the transcription factor hepatocyte nuclear factor (HNF)-1 beta are associated with maturity-onset diabetes of the young (type V), non-diabetic renal disease, and occasionally genital malformations in females. Recently, familial hypoplastic glomerulocystic kidney disease (GCKD) has been added to the clinical spectrum of HNF-1 beta gene mutations. Familial hypoplastic GCKD is a rare, dominantly inherited disorder characterized by small kidneys containing glomerular cysts, abnormal pelvicalyceal anatomy, and chronic renal failure. A family with hypoplastic GCKD occurring in the father and the daughter was screened for mutations in the HNF-1 beta gene. The sequence of exon 4 of the HNF-1 beta gene revealed a C insertion at codon 334 resulting in a frameshift mutation (P334fsinsC) in two family members. The P334fsinsC allele co-segregated with hypoplastic GCKD in the family. Oral glucose tolerance testing was normal in the 11-year-old girl. In her 38-year-old father, impaired glucose tolerance was detected. These studies provide further evidence that familial hypoplastic GCKD is associated with HNF-1 beta gene mutations. HNF-1 beta gene mutation screening may prove useful in patients with small cystic kidneys and chronic renal failure, in whom a definite renal diagnosis could otherwise only be established by renal biopsy.